1. Field of the Invention
This invention relates to a process whereby calcium magnesium salts of acetic acid and other organic acids are produced when biomass, organic waste material and other organic carbonaceous matter is anaerobically digested by a mixed microbial population in the presence of a calcium-magnesium oxygen containing compound.
2. Description of the Prior Art
It is well documented that the de-icing agents currently used, primarily sodium chloride, or rock salt, and smaller quantities of calcium chloride, pose serious health, environmental, and corrosion problems. Sodium chloride and calcium chloride are commonly used because they are the least expensive road de-icing agents available. Sodium chloride, or rock salt, is particularly attractive from a cost standpoint because it exhibits high de-icing efficiency, and it occurs naturally in a substantially pure state and in great abundance. Although the cost of buying and using sodium chloride as a road de-icing agent is exceedingly low, a recent U.S. Environmental Protection Agency study estimated that damage to vehicles, highway structures, flora, fauna and water supplies, inflates the total cost of using rock salt to approximately 14 times the actual cost of mining, distribution and application.
Alternative de-icing agents which are both inexpensive and environmentally safe have been studied. Calcium magnesium acetate (CMA) has been studied and considered by Federal and state agencies as an alternative to sodium chloride or calcium chloride de-icing agents. Laboratory data indicate that CMA has ice melting characteristics similar to those of chloride salts. Because the acetate is biodegradable and calcium and magnesium tend to precipitate in the environment, CMA has fewer potential detrimental health and environmental effects and is also far less corrosive than chloride salts.
Large scale production of CMA is not currently economically feasible. The simplest method of producing CMA is to react dolomitic lime with synthetic acetic acid derived from gas or oil. Synthetic acetic acid is expensive and sufficient quantities are not currently available. For example, if CMA were to penetrate 10 percent of the present rock salt market, a yearly production of 1.5 million tons would be required. Production of 1.5 million tons of CMA would require 1.2 million tons of acetic acid and 600,000 tons dolomitic lime. Dolomitic lime is readily available and relatively inexpensive, but 1.2 million tons of synthetic acetic acid represents twice the 1981 annual production of synthetic acetic acid in the United States. The cost of producing CMA from synthetic acetic acid is approximately 11 to 13 times the cost of rock salt.
In research supported by the Federal Highway Administration and numerous state highway agencies, SRI International developed a fermentation process for the production of CMA. Marynowski, C. W., et al, Process Development for Production of Calcium Magnesium Acetate (CMA); Phase I, SRI International, Report FHWA/RD-82/145, March 1983 and U.S. Department of Transportation, Federal Highway Administration solicitation for study "CMA Manufacture (II); Improved Bacterial Strain for Acetate Production" DTFH61-83-R-00124, June 10, 1983. According to this process, cellulosic matter comprising C.sub.5 and C.sub.6 sugars is fermented using the bacteria Clostridium thermoaceticum. Dolomitic lime is added to the fermentation medium forming calcium and magnesium ions. The cellulosic matter is converted to acetic acid, which reacts with calcium and magnesium ions in situ to form CMA.
This method of producing of CMA, although in theory less expensive than previous methods, suffers serious drawbacks. The fermentation requires a pure culture of C. thermoaceticum, which is extremely difficult to maintain. Specialized culture conditions are required to maintain culture purity and to prevent the development of undesirable mutant strains. Even when costly precautionary measure are taken, contamination of the pure bacterial culture by hardier competing bacteria is common.
In addition to the difficulty of the fermentation process itself, the type of raw feed material which can be used for the fermentation is limited and the feedstock must be pretreated or predigested to liberate the C.sub.5 and C.sub.6 sugars. Ordinarily, this pretreatment entails wet-milling and/or saccharification. Grain, particularly corn, is the preferred feedstock for the fermentation. The cost of the feedstock and its pretreatment contributes significantly to the overall cost of the process. Moreover, C. thermoaceticum bacterial populations cannot withstand as low a pH range as is desirable and cannot tolerate the high concentrations of magnesium, calcium and acetate ions which are required to maintain high acetic acid productivity, yield and selectivity. Research is underway to develop a mutant strain of the bacteria Clostridium thermoaceticum which can produce high yields of acetic acid under conditions of low pH, and high calcium, magnesium and acetate ion concentrations in the fermentation medium.
A series of patents relating to the production and composition of calcium magnesium acetates and calcium acetates for use as road de-icers has issued recently. Various methods of producing calcium magnesium acetate and calcium acetate are taught in U.S. Pat. No. 4,444,672, teaching a chemical, not biological process for manufacturing a calcium acetate salt and teaching the addition of an inert anti-slip solid; U.S. Pat. No. 4,425,251, disclosing mixed dry chemical formulations of a stable acid-acetate salt and a chemical base which reacts when it contacts snow or ice; U.S. Pat. No. 4,400,285, teaching a chemical composition of calcium oxide and calcium magnesium acetate as a de-icing agent; U.S. Pat. No. 4,389,323, teaching a chemical, not biological process for manufacturing two different grades of CMA; U.S. Pat. No. 4,430,242, disclosing a chemical, not biological process for producing calcium acetate and calcium acid-acetate in a rotary kiln-type reactor, with unreacted raw limestone serving as a traction agent; U.S. Pat. No. 4,426,308, teaching a chemical, not biological process for producing calcium acetate containing solution and its conversion to solid flakes; and U.S. Pat. No. 4,377,488, teaching a chemical, not biological process for making calcium acetate de-icing agents.
U.S Pat. No. 4,405,717 teaches the recovery of acetic acid from dilute aqueous solutions. This patent also teaches the production of calcium acetate by the fermentation of glucose using a pure culture of Clostridium thermoaceticum.
British Pat. No. 164,366 teaches the fermentation of cereals in the presence of calcium carbonate to form calcium acetate. British Pat. Nos. 572,664 and 586,282 both describe fermentation in the presence of calcium carbonate to form a water soluble acetate. U.S. Pat. No. 4,430,240 teaches the production of calcium and magnesium acetates from waste products, such as sawdust, by reaction with alkaline earth metal oxides or hydroxides at elevated pressure and temperature.
Two phase anaerobic digestion processes in which biomass is converted to methane gas are taught in U.S. Pat. Nos. 4,022,665 and 4,318,993. In the first stage, or "acid phase" digestion, carbonaceous biomass or organic waste material is converted to primarily soluble volatile fatty acids with the formation of some product gas.